It'd be awesome to be able to mark up OpenStreetMap on paper, while walking around, and still have the benefit of something that others could trace when you're done. Right now, you can throw a GPS in your bag and upload your traces for others to turn into actual road data. If you mark your notes on paper, though, it's a more tedious and lonely process to get that data into the system. For U.S. users, most of the raw roads are in the system already. Largely, what's left to be done consists of repositioning sloppy TIGER/Line roads and adding local detail like parks, schools, zoning, and soon addresses.

It'd be interesting for generated printouts of OSM data to encode enough source information to reconnect the scanned, scribbled-on result back with its point of origin, and use it as an online base map just like GPS traces and Yahoo aerial imagery.

Maybe you'd print out a bunch of them, one for each neighborhood where you'll be walking and making local notes. You could fold them up and stick them in your pocket, photocopy them for other people, whatever. When you were done, you'd scan them in and a web service would figure out where the map came from, and make it available as a traceable base layer in Potlatch or JOSM.

They don't have to be gargoyles, they just have to look unique enough so you could tell them apart and recognize them separately from the map. SIFT can look at an original image, and find features in it like this:

Check out the examples in Lowe's paper - he describes how detected features can be used to reconstruct entire objects.

When a new scan is uploaded, SIFT finds all three gargoyles and then it knows exactly where the corners of the original image are, even if the person scanning it did it upside or sideways. It also knows where to find the QR barcode, the square thing that looks like static in the top left corner. Aaron's been thinking about QR codes for at least two years now, and he's got a simple decoder ring webserver that he banged together out of Java. The code would say something simple like "this map came from 37d48'N, 122d16'W", and that would be just about enough to make the scan into a base map that people could trace in an OpenStreetMap editor.

I got an implementation of SIFT from Andrea Vedaldi and Brian Fulkserson, computer vision students at UCLA. Their implementation makes .sift files like this one or this one. The code to read them isn't over complicated once you know what all the numbers mean - here's a short script, markup.py, that reads the first four values from each line of a .sift file (x, y, scale, rotation) and draws a little circle on the source image. Easy. The remaining 128 numbers on each line are like a signature, you can determine whether features from two different images are the same thing by looking at a simple squared, summed distance between the two.